How does age affect the singing voice? Let's just assume for the sake of the argument that one is taking care of one's voice. Obviously, if one isn't, the voice deteriorates anyway. I've heard of opera singers who change classifications in their thirties and beyond, both male and female. This is especially true for basses. I've also read of pop/rock stars who have had to retrain because of changes in their voices. Lzzy Hale is an example: http://lzzyhaleofficial.tumblr.com/post/127637611086/1st-entry-lzzy-20
I'm a male entering my mid-thirties, and I haven't noticed a change in my range (both lower and upper) or timbre. But I've heard from many voice teachers that both males and females can have a second "voice-puberty" at my age.